16 research outputs found
Fluidity Onset in Graphene
Viscous electron fluids have emerged recently as a new paradigm of
strongly-correlated electron transport in solids. Here we report on a direct
observation of the transition to this long-sought-for state of matter in a
high-mobility electron system in graphene. Unexpectedly, the electron flow is
found to be interaction-dominated but non-hydrodynamic (quasiballistic) in a
wide temperature range, showing signatures of viscous flows only at relatively
high temperatures. The transition between the two regimes is characterized by a
sharp maximum of negative resistance, probed in proximity to the current
injector. The resistance decreases as the system goes deeper into the
hydrodynamic regime. In a perfect darkness-before-daybreak manner, the
interaction-dominated negative response is strongest at the transition to the
quasiballistic regime. Our work provides the first demonstration of how the
viscous fluid behavior emerges in an interacting electron system.Comment: 8pgs, 4fg
Giant magnetoresistance of Dirac plasma in high-mobility graphene
The most recognizable feature of graphene's electronic spectrum is its Dirac
point around which interesting phenomena tend to cluster. At low temperatures,
the intrinsic behavior in this regime is often obscured by charge inhomogeneity
but thermal excitations can overcome the disorder at elevated temperatures and
create electron-hole plasma of Dirac fermions. The Dirac plasma has been found
to exhibit unusual properties including quantum critical scattering and
hydrodynamic flow. However, little is known about the plasma's behavior in
magnetic fields. Here we report magnetotransport in this quantum-critical
regime. In low fields, the plasma exhibits giant parabolic magnetoresistivity
reaching >100% in 0.1 T even at room temperature. This is orders of magnitude
higher than magnetoresistivity found in any other system at such temperatures.
We show that this behavior is unique to monolayer graphene, being underpinned
by its massless spectrum and ultrahigh mobility, despite frequent
(Planckian-limit) scattering. With the onset of Landau quantization in a few T,
where the electron-hole plasma resides entirely on the zeroth Landau level,
giant linear magnetoresistivity emerges. It is nearly independent of
temperature and can be suppressed by proximity screening, indicating a
many-body origin. Clear parallels with magnetotransport in strange metals and
so-called quantum linear magnetoresistance predicted for Weyl metals offer an
interesting playground to further explore relevant physics using this
well-defined quantum-critical 2D system.Comment: 8 pages, 3 figure
Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor
Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristicâthe topological surface statesâhas proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spinâorbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor
Magnetization Signature of Topological Surface States in a Non-Symmorphic Superconductor
Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristicâthe topological surface statesâhas proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the non-symmorphic crystal symmetry and strong spinâorbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a near-perfect, Meissner-like screening of low-frequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductor
Scanning SQUID-on-tip microscope in a top-loading cryogen-free dilution refrigerator
The scanning superconducting quantum interference device (SQUID) fabricated
on the tip of a sharp quartz pipette (SQUID-on-tip) has emerged as a versatile
tool for nanoscale imaging of magnetic, thermal, and transport properties of
microscopic devices of quantum materials. We present the design and performance
of a scanning SQUID-on-tip microscope in a top-loading probe of a cryogen-free
dilution refrigerator. The microscope is enclosed in a custom-made vacuum-tight
cell mounted at the bottom of the probe and is suspended by springs to suppress
vibrations caused by the pulse tube cryocooler. Two capillaries allow in-situ
control of helium exchange gas pressure in the cell that is required for
thermal imaging. A nanoscale heater is used to create local temperature
gradients in the sample, which enables quantitative characterization of the
relative vibrations between the tip and the sample. The spectrum of the
vibrations shows distinct resonant peaks with maximal power density of about 27
nm/Hz in the in-plane direction. The performance of the SQUID-on-tip
microscope is demonstrated by magnetic imaging of the MnBiTe magnetic
topological insulator, magnetization and current distribution imaging in a
SrRuO ferromagnetic oxide thin film, and by thermal imaging of dissipation
in graphene.Comment: Submitted to Review of Scientific Instrument
Magnetization Signature of Topological Surface States in a NonâSymmorphic Superconductor
From Wiley via Jisc Publications RouterHistory: received 2021-04-28, rev-recd 2021-06-16, pub-electronic 2021-08-08Article version: VoRPublication status: PublishedFunder: European Union's Horizon 2020 research and innovation programme; Grant(s): 785219, 881603Funder: EPSRC; Id: http://dx.doi.org/10.13039/501100000266; Grant(s): EP/L01548XAbstract: Superconductors with nontrivial band structure topology represent a class of materials with unconventional and potentially useful properties. Recent years have seen much success in creating artificial hybrid structures exhibiting the main characteristics of 2D topological superconductors. Yet, bulk materials known to combine inherent superconductivity with nontrivial topology remain scarce, largely because distinguishing their central characteristicâthe topological surface statesâhas proved challenging due to a dominant contribution from the superconducting bulk. In this work, a highly anomalous behavior of surface superconductivity in topologically nontrivial 3D superconductor In2Bi, where the surface states result from its nontrivial band structure, itself a consequence of the nonâsymmorphic crystal symmetry and strong spinâorbit coupling, is reported. In contrast to smoothly decreasing diamagnetic susceptibility above the bulk critical field, Hc2, as seen in conventional superconductors, a nearâperfect, Meissnerâlike screening of lowâfrequency magnetic fields well above Hc2 is observed. The enhanced diamagnetism disappears at a new phase transition close to the critical field of surface superconductivity, Hc3. Using theoretical modeling, the anomalous screening is shown to be consistent with modification of surface superconductivity by the topological surface states. The possibility of detecting signatures of the surface states using macroscopic magnetization provides a new tool for the discovery and identification of topological superconductors
Polarized blazar X-rays imply particle acceleration in shocks
Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1âTeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarizationâthe only range available until nowâprobe extended regions of the jet containing particles that left the acceleration site days to years earlier1,2,3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree Î X of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock
X-ray Polarization Observations of BL Lacertae
Blazars are a class of jet-dominated active galactic nuclei with a typical
double-humped spectral energy distribution. It is of common consensus the
Synchrotron emission to be responsible for the low frequency peak, while the
origin of the high frequency hump is still debated. The analysis of X-rays and
their polarization can provide a valuable tool to understand the physical
mechanisms responsible for the origin of high-energy emission of blazars. We
report the first observations of BL Lacertae performed with the Imaging X-ray
Polarimetry Explorer ({IXPE}), from which an upper limit to the polarization
degree 12.6\% was found in the 2-8 keV band. We contemporaneously
measured the polarization in radio, infrared, and optical wavelengths. Our
multiwavelength polarization analysis disfavors a significant contribution of
proton synchrotron radiation to the X-ray emission at these epochs. Instead, it
supports a leptonic origin for the X-ray emission in BL Lac.Comment: 17 pages, 5 figures, accepted for publication in ApJ
Electronic phase separation in multilayer rhombohedral graphite
Of the two stable forms of graphite, hexagonal (HG) and rhombohedral (RG),
the former is more common and has been studied extensively. RG is less stable,
which so far precluded its detailed investigation, despite many theoretical
predictions about the abundance of exotic interaction-induced physics. Advances
in van der Waals heterostructure technology have now allowed us to make
high-quality RG films up to 50 graphene layers thick and study their transport
properties. We find that the bulk electronic states in such RG are gapped and,
at low temperatures, electron transport is dominated by surface states. Because
of topological protection, the surface states are robust and of high quality,
allowing the observation of the quantum Hall effect, where RG exhibits phase
transitions between gapless semimetallic phase and gapped quantum spin Hall
phase with giant Berry curvature. An energy gap can also be opened in the
surface states by breaking their inversion symmetry via applying a
perpendicular electric field. Moreover, in RG films thinner than 4 nm, a gap is
present even without an external electric field. This spontaneous gap opening
shows pronounced hysteresis and other signatures characteristic of electronic
phase separation, which we attribute to emergence of strongly-correlated
electronic surface states.Comment: 18 pages, 12 figure
Out-of-equilibrium criticalities in graphene superlattices
In thermodynamic equilibrium, current in metallic systems is carried by electronic states near the Fermi energy, whereas the filled bands underneath contribute little to conduction. Here, we describe a very different regime in which carrier distribution in graphene and its superlattices is shifted so far from equilibrium that the filled bands start playing an essential role, leading to a critical-current behavior. The criticalities develop upon the velocity of electron flow reaching the Fermi velocity. Key signatures of the out-of-equilibrium state are current-voltage characteristics that resemble those of superconductors, sharp peaks in differential resistance, sign reversal of the Hall effect, and a marked anomaly caused by the Schwinger-like production of hot electron-hole plasma. The observed behavior is expected to be common to all graphene-based superlattices